There are a large number of processes for hydrocracking petroleum hydrocarbon feedstocks and numerous catalysts that are used in these processes. Many of these processes comprise two stages, a feed preparation stage and a hydrocracking stage, the two stages operating with different catalysts. The first stage, in general, contains a hydrodenitrogenation/hydrodesulfurization catalyst which also may include a hydrocracking function for mild hydrocracking and the second stage contains a hydrocracking catalyst. Product from the first stage may be treated to remove ammonia and hydrogen sulfide gases prior to being passed to the second stage, or product may be passed directly to the second stage. In this two stage operation, the hydrocracking stage is frequently referred to as a second stage hydrocracker.
Multiple beds in a hydrocracker have been disclosed in U.S. Pat. Nos. 4,797,195; 4,797,196 and 4,834,865. The latter patent also discloses the use of two different sizes of catalyst in a hydrocracker.
Hydrocracking catalysts generally comprise a hydrogenation component on an acidic cracking support. More specifically, hydrocracking catalysts comprise one or more hydrogenation components selected from the group consisting of Group VIB metals and Group VIII metals of the Periodic Table of the Elements, their oxides or sulfides. The prior art has also taught that these hydrocracking catalysts preferably contain an acidic support comprising a large pore crystalline molecular sieve, particularly an aluminosilicate. These molecular sieves are generally suspended in a refractory inorganic oxide binder such as silica, alumina, or silica-alumina. The oxides such as silica, silica-alumina and alumina have also been used alone as the support for the hydrogenating metals for certain specific operations.
Regarding the hydrogenation component, the preferred Group VIB metals are tungsten and molybdenum and the preferred Group VIII metals are nickel and cobalt. The prior art has also taught that combinations of metals for the hydrogenation component in the order of preference are: Ni-W, Ni-Mo, Co-Mo and Co-W. Other hydrogenation components broadly taught by the prior art include iron, ruthenium, rhodium, palladium, osmium, iridium and platinum. Among these latter components, platinum and/or palladium are particularly preferred with palladium being most preferred.
Hydrocracking is a general term which is applied to petroleum refining processes wherein hydrocarbon feedstocks which have relatively high molecular weights are converted to lower molecular weight hydrocarbons at elevated temperature and pressure in the presence of a hydrocracking catalyst and a hydrogen-containing gas. Hydrogen is consumed in the cracking of the high molecular weight compounds to lower molecular weight compounds. Hydrogen will also be consumed in the conversion of any organic nitrogen and sulfur compound to ammonia and hydrogen sulfide as well as in the saturation of olefins and other unsaturated compounds. The hydrocracking reaction is exothermic and when substantially adiabatic reactors are used, as is usually the case, the temperature in the catalyst bed will rise progressively from the beginning to the end of the reactor. Excessive temperature in the reactor can present several problems. High temperatures can damage the catalyst, can result in the safe operating temperature of the reactor being exceeded or can cause the hydrocracking reaction to "run away", with disastrous results. This temperature rise problem can be solved by dividing the catalyst in the reactor into a series of beds with interstage cooling supplied between the beds by the injection of a cooled hydrogen-containing gas stream.
When the multiple bed configuration is used in a second stage hydrocracker, optimum use of the catalyst requires that each bed do a proportionate amount of the hydroconversion. For example in the common five bed second stage hydrocracker each bed should carry out about twenty percent of the hydroconversion, resulting in a temperature rise in each of the beds of about the same degree. It has been found, however, that in many cases the catalyst in the first bed is somehow inhibited such that its activity is less than that of the catalyst in the remaining beds. As a result, the first bed carries out less than its proportionate share of hydroconversion, thus resulting in a smaller temperature rise in the first bed than occurs in the remaining beds. Raising the temperature of the feed to the first bed can increase conversion, but can also require excessive cooling between the first and second bed which will result in an inefficient utilization of hydrogen. Further, if the physical configuration of the reactor limits the amount of hydrogen that can be injected between the beds or limits the temperature to which the top bed can be heated, then the top bed can not be operated at its full hydroconversion potential. It has been found that by modifying the catalyst in the first bed over that in the remaining beds pursuant to the teachings of the instant invention by providing it with higher hydrogenation metals content and smaller particle size, the conversion in the first bed can be raised to the level in the remaining beds, resulting in a more efficient operation.